Isolated gate devices, such as metal oxide semiconductor field-effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs), and metal oxide semiconductor (MOS) -controlled thyristors (MCTs) , are commonly found in many diverse electronic applications. One application of isolated gate devices, is in electronic power conversion circuits, such as those found in telecommunications power systems, motor controllers and uninterruptable power supplies (UPS).
Fast switching speed, ease of drive, wide safe operating area (SOA), peak current capability, avalanche and high dv/dt capability have made power MOSFETs a logical choice in modern power conversion circuits. IGBTs, on the other hand, being minority carrier MOS gate devices, have excellent conduction characteristics, while sharing many of the appealing features of power MOSFETs, such as ease of drive, wide SOA, peak current capability and ruggedness. Both IGBTs and MOSFETs, therefore, are widely used in electronic power conversion circuits.
The development of high-efficiency power converters with higher power density is a continuing goal in the field of power electronics. To reduce energy storage component size and to increase power density, switching frequencies of the isolated gate devices in the power conversion circuits are pushed higher and higher. One advantage of the high switching frequency is a reduction in switching power loss due to a reduction in a voltage-current crossover time. The rate of change of a drain-source voltage and a drain current of a MOSFET device (or similarly, the rate of change of a collector-emitter voltage and a collector current of an IGBT device), and thereby the switching frequency, is dependent on a gate current, which limits a charge and discharge rate of device capacitances. In high frequency applications, the gate current of the device may be substantial, thereby significantly increasing switching time.
It has been determined that the application of a negative bias voltage during a turn-off transition (negative off-bias voltage) significantly reduces the switching time and improves the immunity to spurious gate voltages of the switch. A conventional low side gate drive circuit, having a dedicated power supply, is typically used to provide the negative off-bias voltage to the switch. The gate drive circuit further includes a totem pole driver consisting of an npn transistor serially-coupled to a pnp transistor. The gate drive circuit further includes a resistor-zener structure coupled across the totem pole driver and powered by the dedicated power supply. The resistor-zener structure consists of a current-limiting resistor coupled in series with a zener diode. An energy storage capacitor is typically coupled across the zener diode to provide for local energy storage. The gate drive circuit still further includes a gate damping resistor coupled between the totem pole driver and a gate of an isolated gate device.
The gate drive circuit operates as follows. The npn transistor is turned on to apply a positive voltage to the gate of the isolated gate device often through the gate damping resistor, thereby turning on the isolated gate device. To turn off the isolated gate device, the npn transistor is turned off and the pnp transistor is turned on. The negative off-bias voltage stored in the energy storage capacitor by the zener diode is thus applied to the gate of the isolated gate device, quickly discharging a gate capacitance of the isolated gate device. While the above approach fulfills its intended purpose, a dedicated power supply is required, which not only increases system cost, but also consumes valuable space on the printed wiring board.
Accordingly, what is needed in the art is a gate drive circuit that provides its own negative off-bias voltage, removing the need for a dedicated power supply to provide the negative off-bias voltage.